Broken Access Control in Echo Go with Hmac Signatures

Broken Access Control occurs when an API fails to enforce proper authorization checks, allowing one user to act as another. In Echo Go, combining route-level authorization decisions with Hmac Signatures can inadvertently create or expose access control weaknesses if the signature is used only for integrity/authenticity and not bound to the intended recipient or scope.

Hmac Signatures provide integrity and authenticity by signing a canonical representation of the request (method, path, selected headers, and body). In Echo Go, developers commonly compute an Hmac over a subset of request data and then verify the signature before routing to a handler. If the verification passes but the handler does not re-check authorization (e.g., which resources a user may access), the request can proceed with elevated or unintended permissions. This is a BOLA/IDOR pattern where valid authentication (Hmac) does not equate to proper authorization.

Consider an endpoint like /api/users/{userId}/profile. The Hmac is computed over the URL path and body, but if the server only verifies the signature and then fetches the profile using userId from the URL without confirming that the authenticated subject is allowed to access that specific user, a horizontal privilege escalation occurs. An attacker who knows or guesses another user’s ID can read or modify profiles because the signature mechanism does not encode or enforce ownership or role constraints.

Echo middleware can verify the Hmac, but if the authorization check is omitted or applied inconsistently across routes, the attack surface remains. For example, using different shared secrets per client without scoping the signed payload to the intended operation enables a BFLA/Privilege Escalation vector where a lower-privilege key is reused to reach higher-privilege endpoints. Additionally, if the signature includes only the path and not query parameters that alter behavior (such as admin flags), an attacker can manipulate unsigned parameters after a valid signature is generated to trigger unauthorized actions.

Real-world parallels exist in API designs where Hmac is used for request signing in services like AWS Signature Version 4, which binds scope and conditions to the signature. If Echo Go applications replicate signing patterns but omit scope binding, condition checks, or per-request nonces, they risk Insecure Direct Object References and Missing Function Level Access Control. The combination of a fast, unauthenticated scan by middleBrick and the presence of Hmac Signatures can surface these authorization gaps through property-level authorization checks and BOLA testing, revealing endpoints that trust the signature but skip ownership validation.

To remediate Broken Access Control when using Hmac Signatures in Echo Go, bind the signature verification to authorization logic and ensure the signed payload explicitly includes scope, subject, and intended action. Always verify the signature before processing, then enforce authorization based on the claims encoded in the signed data or via additional server-side checks.

Example: include userId and a scope claim inside the signed string, and validate ownership in the handler.

package main

import (
	"crypto/hmac"
	"crypto/sha256"
	"encoding/hex"
	"net/http"
	"github.com/labstack/echo/v4"
)

// computeHmac returns hex-encoded Hmac-SHA256 of message using key
func computeHmac(message string, key string) string {
	h := hmac.New(sha256.New, []byte(key))
	h.Write([]byte(message))
	return hex.EncodeToString(h.Sum(nil))
}

// verifyHmac checks whether receivedSig matches computed Hmac
func verifyHmac(message string, receivedSig string, key string) bool {
	expected := computeHmac(message, key)
	return hmac.Equal([]byte(expected), []byte(receivedSig))
}

func main() {
	e := echo.New()

	e.GET("/api/users/:userId/profile", func(c echo.Context) error {
		userId := c.Param("userId")
		receivedSig := c.Request().Header.Get("X-API-Signature")
		apiKey := c.Request().Header.Get("X-Client-Key") // map clientKey to userId or role as needed

		// Build canonical payload including the intended action and scope
		canonical := "GET" + "
" + "/api/users/" + userId + "/profile" + "
" + "userId=" + userId + "
" + "scope=read:own_profile"

		if !verifyHmac(canonical, receivedSig, apiKey) {
			return c.JSON(401, map[string]string{"error": "invalid signature"})
		}

		// Authorization: ensure the requesting subject can access this userId
		// In practice, resolve apiKey to an actor, then compare actor.owns(userId) or actor.canRead(userId)
		if !isAuthorized(c, userId) {
			return c.JSON(403, map[string]string{"error": "access denied"})
		}

		return c.JSON(200, map[string]string{"profile": "data"})
	})

	e.Logger.Fatal(e.Start(":8080"))
}

// isAuthorized is a placeholder for your domain logic (e.g., role/relationship checks)
func isAuthorized(c echo.Context, targetUserId string) bool {
	// Example: extract subject from context after identity resolution
	subject, ok := c.Get("actor").(string)
	if !ok {
		return false
	}
	// Allow access only if subject owns the userId or has explicit permission
	return subject == targetUserId || hasRole(c, "admin")
}

func hasRole(c echo.Context, role string) bool {
	// Implement role resolution from claims or token
	return false
}

Key remediation practices:

• Include scope and subject in the signed payload to bind the signature to the intended operation and resource.
• Always perform a server-side authorization check after signature verification; do not rely on the signature alone for access control.
• Use constant-time comparison (e.g., hmac.Equal) to avoid timing attacks on the signature.
• Map the Hmac’s client key to a principal and enforce ownership or role-based checks consistent with the application’s access control model.
• If multiple endpoints share a key, encode the intended operation (HTTP method + path) into the signed string to prevent privilege escalation across routes.

These steps align with OWASP API Security Top 10 controls for Broken Object Level Authorization and ensure that Hmac Signatures enforce integrity while server-side logic enforces authorization.